At the recent Irish Association for Cancer Research (IACR) conference in Cork, a session devoted to microbes and cancer featured speakers from Oxford, Harvard, and Cork, describing clinical and preclinical research on oncolytic viruses, cancer-associated bacteria, and bacterial-mediated cancer therapy, respectively. 

My lab’s work at the Cork Cancer Research Centre (CCRC) on microbes and cancer spans from basic research, such as investigating the bacterial content of patient tumours to technology development, basically the exploitation of microbes (bacteria or oncolytic viruses) for cancer treatment or diagnosis. 

What’s already here?

Live microbes are already safely used clinically in cancer treatment. Bacillus Calmette-Guerin (BCG) is the main intravesical immunotherapy for treating superficial bladder cancer, and consists of live Mycobacterium bacteria. The US FDA approved the use of BCG for superficial bladder cancer in 1990, and it is in several late-stage combination therapy clinical trials for other cancers. 

Talimogene laherparepvec, also known as T-Vec, was approved by the FDA in 2015 for the treatment of advanced inoperable melanoma, with the brand name Imlygic. In January of this year, it was approved in Europe. It is an oncolytic virus and consists of a genetically modified herpes simplex virus (HSV) carrying the gene for a cytokine that helps to induce immune responses following intralesional injection. So the precedent is set, but the best is yet to come.

What’s on the way?

Oncolytic viruses

Both of the above drugs are immunotherapies, as their primary activity relates to recruitment of tumour-specific immune responses. However, oncolytic viruses such as T-Vec are in fact directly cytotoxic to cancer cells, with immune recruitment secondary to direct cell killing.¹

Oncolytic virus research originated on the premise that mutations within cancer cells facilitating uncontrolled cell division and malignancy also have the effect of making cancer cells less well able to inhibit viral infection, thereby rendering a cancer cell highly susceptible to destruction by viral replication, much more so than a healthy cell. 

Engineering of viruses further facilitates reduction in viral replication in healthy cells (safer) and increased replication in cancer cells (increased cancer cell killing), thereby increasing the therapeutic index. The ability to insert genes to the virus, coding for agents such as immune upregulating molecules (eg. cytokines), results in a powerful multi-pronged therapeutic platform.²

Beyond T-Vec, there are multiple oncolytic viral agents in development and in late-stage clinical trials with various indications, which are likely to appear on our shelves shortly.

Bacteria and tumours

Bacteria have been linked with various cancers in a number of ways. For example, local bacterial-induced inflammation has been linked with cancer promotion and progression directly, such as in the case of Helicobacter pylori.³ Specific, individual bacterial species have been isolated from a number of tumour types.³ Such studies have analysed patient tumours for the presence of a given specific strain, in an attempt to correlate the presence of this bacterium with cancer causation.

The microbiome story

The growing body of evidence supporting associations between the human microbiome and our health has drawn significant attention. With respect to cancer, associations between the nature of cancer patients’ gut microbiota and tumour progression have been established.⁴ Recent research in experimental tumours has revealed that gut bacteria may influence the outcome of chemotherapy or immunotherapy indirectly via influencing the immune system.^5,6

Bacterial growth in tumours

For decades, naturally occurring bacteria of different types have been isolated from patient tumours of various histological types.³ However, we need to consider whether bacterial presence in tumours reflects a causative relationship (bacterial presence in healthy tissue induced malignancy?), or an opportunistic infection of pre-existing tumour tissue? 

Work in our lab has shown that tumours support the growth of multiple bacterial species. Various bacteria preferentially colonise and proliferate within tumours following systemic administration, as demonstrated by us and others in preclinical and clinical tumour models. 

Furthermore, we reported in 2014 the presence of a wide range of bacteria in patient breast tumours, and the concept of a ‘tumour microbiome’ was conceived.⁷

Factors responsible for the phenomenon of intratumoural bacterial growth include disorganised tumour vasculature which allows entry of bacteria into the tumour; local immunosuppression which prevents bacterial elimination by the host immune response; rich nutrients within necrotic tumour regions; as well as low oxygen (in which relevant bacteria are capable of growing), all representing factors peculiar to tumour tissue over healthy tissue.³

Exploitation in cancer medicine

Tumour-specific bacterial growth presents opportunities for cancer therapy or diagnosis. Bacteria may be exploited to:

• Specify therapeutic action to a tumour 

• Act as a read-out for the presence of a tumour. 

Our lab has investigated exploitation of bacteria in both strategies, using bacteria with and without genetic modification.

In particular, we have focused on probiotic bacteria.⁸ Until recently, genetic tools for many probiotic bacteria were poor. Unique expertise with these types of bacteria in UCC has permitted us to develop a range of genetic platforms for tumour-specific production of heterologous proteins (eg. for immunotherapy).^9,10 Recently, we have also reported the ability of non-engineered probiotics to mediate effective tumour therapy in conjunction with pro-drug chemotherapies.^11,12

While intravenous or intralesional administration is typically employed in these strategies, we were the first to show that oral administration is applicable to targeting distal tumours with certain probiotic bacterial species. An interesting diagnostic application of this approach was recently described by researchers from MIT in Science Translational Medicine where engineering of the orally administered probiotic facilitated detection of hepatic tumours in mice via analysis of urine samples.¹³ Overall, this platform technology is applicable to a wide range of therapeutic or diagnostic strategies. 

Cancer vaccines employing bacteria

The goal of cancer vaccines is to break tolerance of the immune system to specific antigens known to be expressed mainly or exclusively by particular tumour cells – tumour-associated antigens (TAA). Used in this setting, the bacterium acts as a vehicle to deliver an antigen-to-antigen presenting cells (APC), such as M cells in the gut mucosa, and does not involve growth in tumours. A number of companies are developing drugs involving engineering of pathogenic bacteria such as Listeria monocytogenes, to become therapeutic agents that stimulate targeted immune responses to specific tumour antigens. The bacterium is safety attenuated to render it non-infectious, and engineered to produce specific tumour antigens (either genes or proteins). 

The bug itself also induces a desirable immune response in the vaccine context (similar to an adjuvant). Following administration (per oral, intramuscular or intravenous), the bacterium is taken up by the patient’s antigen presenting cells, including dendritic cells, which are primary initiators of both the innate and adaptive immune responses. 

The bacterium releases genetic material or antigens into the immune cells that then initiate a systemic immune response specific to the target antigen. Such vaccine drugs are proving safe and effective in our laboratory¹⁴ and more importantly in late stage clinical trials with various indications.¹⁵

What will happen next?

Dramatic advances in genetic engineering under the synthetic biology (SB) umbrella offer unprecedented new opportunities to design and build novel, more rapidly developed medicines. Recent advances in enabling technologies, combined with already available extensive genome knowledge, are significantly reducing SB cost barriers, and the resulting Biotech 2.0 is leading an ICT-style revolution due to shorter innovation cycles, lower costs and new market opportunities. 

Consequently, significant opportunity exists over the next decade to create next-generation customised biomolecules for use in the treatment of cancer and other diseases. To realise this potential, forward-thinking Irish scientists involved in cancer research must embrace (at the earliest of R&D stages) the various stakeholders in the drug development chain – entrepreneurs, venture capitalists, big pharma, the regulators, and not least, clinicians. 

We, as scientists, possess a genuine desire for our lives’ work to impact cancer patients in the near-term, and to stand a chance at success, must remain informed that the solutions proposed are needed, and feasible at every step of the development cycle. Together, we can turn bugs into drugs.

Mark Tangney is principal investigator at the Cork Cancer Research Centre,
University College Cork

References

1. Lichty BD, Breitbach CJ, Stojdl DF, Bell JC. Going viral with cancer immunotherapy. Nature reviews Cancer 2014; 14(8): 559-567
2. Tedcastle A, Cawood R, Di Y et al. Virotherapy – cancer targeted pharmacology. Drug discovery today 2012; 17(5-6): 215-220
3. Cummins J, Tangney M. Bacteria and tumours: causative agents or opportunistic inhabitants? Infect Agent Cancer 2013; 8(1): 11
4. Schwabe RF, Jobin C. The microbiome and cancer. Nature reviews Cancer 2013; 13(11): 800-812
5. Viaud S, Daillere R, Boneca IG et al. Gut microbiome and anticancer immune response: really hot Sh*t! Cell death and differentiation 2015; 22: 199-214
6. Sivan A, Corrales L, Hubert N et al. Commensal Bifidobacterium promotes antitumor immunity and facilitates anti-PD-L1 efficacy. Science 2015; 350(6264): 1084-1089
7. Urbaniak C, Cummins J, Brackstone M et al. Microbiota of human breast tissue. Appl Environ Microbiol 2014; 80(10): 3007-3014
8. Tangney M. Gene therapy for cancer: dairy bacteria as delivery vectors. Discov Med 2010; 10(52): 195-200
9. Cronin M, Le Boeuf F, Murphy C et al. Bacterial-mediated knockdown of tumor resistance to an oncolytic virus enhances therapy. Mol Ther 2014; 22(6): 1188-1197
10. Byrne WL, Murphy CT, Cronin M, Wirth T, Tangney M. Bacterial-mediated DNA delivery to tumour associated phagocytic cells. J Control Release 2014; 196: 384-393
11. Lehouritis P, Stanton M, McCarthy FO, Jeavons M, Tangney M. Activation of multiple chemotherapeutic prodrugs by the natural enzymolome of tumour-localised probiotic bacteria. J Control Release 2016; 222: 9-17
12. Lehouritis P, Cummins J, Stanton M et al. Local bacteria affect the efficacy of chemotherapeutic drugs. Sci Rep 2015; 5:14554
13. Danino T, Prindle A, Kwong GA et al. Programmable probiotics for detection of cancer in urine. Science Translational Medicine 2015; 7(289): 289ra84
14. Ahmad S, Casey G, Cronin M et al. Induction of effective antitumor response after mucosal bacterial vector mediated DNA vaccination with endogenous prostate cancer specific antigen. J Urol 2011; 186(2): 687-693
15. Wood LM, Paterson Y. Attenuated Listeria monocytogenes: a powerful and versatile vector for the future of tumor immunotherapy. Frontiers in cellular and infection microbiology. 2014; 4:51